The invention relates to a method for producing a component by hot forming a pre-product made of steel according to the preamble of patent claim 1. In the following, pre-products include for example sheet metal plates or seamless pipes or respectively, welded pipes.
Such components are predominantly used in the automobile industry but may also be used in mechanical or civil engineering.
The hotly contested automobile market forces manufacturers to constantly seek solutions for lowering the fleet consumption, while at the same time maintaining a highest-possible comfort and occupant protection. In this context, weight-saving plays an important role, on the other hand also properties of the individual components that promote the passive safety of the passengers during high static and dynamic stresses during operation and in case of a crash.
Suppliers of starting material seek to account for this demand by providing high-strength and ultra-high-strength steels which allow reducing wall thicknesses while at the same time providing improved properties of the components during manufacture and during use.
These steels therefore have to satisfy relatively high standards regarding strength, stretchability, tenacity, energy absorption and corrosion resistance as well as their processability for example during cold forming and during welding.
In light of the aforementioned aspects, the production of components made of hot formable steels is gaining importance because these ideally meet the increased demands placed on the component properties while at the same time requiring less material.
The production of components by means of quenching of pre-products made of press hardenable steels by hot forming in a forming tool is known from DE 601 19 826 12. Here, a sheet metal plate which has been heated above austenizing temperature to 800-1000° C. beforehand and may optionally be covered with a metallic coating on zinc basis, is formed into a component in an optionally cooled tool, wherein during the forming the sheet metal plate or component undergoes a quench hardening (press hardening) by rapid heat withdrawal and as a result attains the required strength properties. The metallic coating is applied as corrosion protection, usually in the continuous hot dip coating, onto a hot strip or cold strip or respectively onto the pre-product produced therefrom, for example as hot dip galvanizing or hot dip aluminizing.
Subsequently, the plate is cut to size for the forming tool in accordance with the hot forming. It is also possible to provide the respective work piece to be formed or the cut with a hot dip coating.
The application of a metallic coating onto the pre-product to be formed prior to the hot forming is advantageous in this method because the coating effectively avoids scaling of the base material, and as a result of the additional lubricating effect, excessive tool wear.
Known hot formable steels for this field of application are for example the manganese-boron steel “22MnB5” and recently also air-hardenable steels according to a not yet published patent application of the applicant.
The production of a component by hot forming by means of the known methods has multiple disadvantages.
On one hand this method requires very high amounts of energy due to the heating of the pre-product to austenizing temperature and the transformation of ferrite into austenite, which renders the method expensive and produces significant amounts of CO2.
In addition, for avoiding excessive scaling of the sheet metal surface as described above an additional metallic protective layer or a protective lacquer based layer is required or a significant post processing of the surface that has undergone scaling as a result of heating and forming.
Because the forming above the AC3-temperature, usually takes place at temperatures significantly above 800° C., extremely high requirements are placed on these layers regarding temperature stability.
A further disadvantage is also that for attaining corresponding strength of the components after the press hardening, only transformation-capable steels with a sufficiently slow transformation can be used which require correspondingly expensive alloy additions for the microstructure and hardness to be achieved after the forming.
In summary, the known method for producing components from steel by forming above austenizing temperature is very cost-intensive due to high energy costs and expensive materials which leads to high prices for components. For improving the forming capacity of high-strength steels it is known from DE10 2004 028 236 B3 to further process work pieces instead by cold forming, by a hot forming at temperatures from 400 to 700° C. to a component (warm forming). It is also disadvantageous in this method in a high component strength can only be obtained by using materials which are of higher strength and with this expensive.